I never understood what causes the leaves and stems to harden off. Is it the increase in photosynthates produced by longer time in the sun, is it the UV rays that causes the cell walls to harden off rather than burn away, or is it the increase in temperture and drop in humidity and they must adapt to if not to lose water, could the wind play a part in forcing the new stems to produce lignin in cell walls. Could all these factors play a role together?

AFB

I have always understood it to be any process or time period when a plant is adapted from one environment to another, such as from a greenhouse to local climate (generally meaning less humidity and more sun) or from shady environment to full sun. Many people in shorter season climates start seedlings indoors (pumpkins, beans, whatever) to give them enough time to produce and ripen a crop, but the plants are not accustomed to full sun, so have to have a transition period and protocol.

Lignification is the process of turning "green" tissue into hardened tissue, a process which is often an important part of a plant's ability to withstand colder temps, and can be initiated by cooling temps, or artificially by witholding water, or perhaps light. Ceasing fertilization at a particular point in the season reduces growth and helps bring on lignification. Fertilizing too late in the season cause the plant to put on a lot of new growth too close to colder weather, which is then unable to tolerate the cold, and consequently die back.